In order to study a pathogenetic roles of pancreatic antigen on beta-cell failure in insulin-dependent diabetes mellitus (IDDM), characterization of autoantibodies to glutamic acid decarboxylase 65 (GAD65), a representative autoantigen in LDDM, was performed.We created chimeric GAD65 and GAD67 protein molecules to examine the epitopes of autoantibodies to GAD65 (GAD65Ab) to different regions of GAD65 molecules.Longitudical changes of the reactivity of GAD65Ab in acute-onset IDDM and slowly progressive IDDM were also studies.The constructed molecules includes ; the chimera GAD65(1-244)/GAD67(253-369)/GAD65(360-585) : [A], the chimera GAD65(1-359)/GAD67(370-451)/GAD65(443-585) : [B], chimera GAD65(1-244)/GAD67(253-594) : [C], chimera GAD65(1-244)/GAD67(253-451)/GAD65(443-585) : [D], the chimera GAD65(1-442)/GAD67(452-594) : [E], GAD67(1-443)/GAD65(451-585) : [H], chimera GAD65(1-83)/ GAD67(89-594) : [N].Sera from the patients with acute-onset IDDM reacted with chimera [B] and [E].However the reactivities with chimera [A] [D], [N] and [H] were reduced significantly, showing the epitope of GAD65Ab reside on amino acid (AA) residues 244-360 and 244-443.Sera from the patients with slowly progressive IDDM reacted with chimera [N], [A], [B], [D], [E] and [H], these reactivity was reduced when incubated with chimera [C].These results shows that GADAb epitope in slowly progressive IDDM resides AA residues 244-585 and 1-83.The reactivity of sera from both acute-onset IDDM and slowly progressive IDDM to chimera GAD molecules did not change 0.5 and 5 years after the onset of diabetes.These results suggest that "epitope spreading" of GAD65 did not occure in both type of IDDM.